A power source apparatus, which includes a first power source circuit, is known in the related art (for example, see patent document 1). The first power source circuit includes a series circuit of a first switching element and a first inductor coupled to a power source and includes a series circuit of a first capacitor and a rectifier element coupled to the first switching element in parallel. A first load is coupled to the first capacitor. The power source apparatus further includes a second power source circuit that is coupled to the first capacitor. The second power source circuit includes a second switching element and converts output voltage to a second load to which electric power is supplied by switching of the second switching element.
When supplying electric power to the first load, the power source apparatus operates only the first switching element to convert the voltage of the power source to supply the electric power to the first load. When supplying electric power to the first load, the power source apparatus does not operate the second switching element and does not supply the electric power to the second load. When supplying electric power to the second load, the power source apparatus operates the second switching element to convert the voltage of the power source to supply the electric power to the second load. When supplying the electric power to the second load, the power source apparatus turns off the first switching element and uses the first inductor and the first capacitor of the first power source circuit to eliminate noise generated in the second power source circuit.
However, in the above power source apparatus, when only one of the first power source circuit and the second power source circuit is operated, the other inductor and capacitor cannot be used. Thus, in order to take measures against the noise in an input unit that inputs electric power to the first power source circuit and the second power source circuit, it is required to apply measures against the noise to the respective power source circuits.